Genes belonging to the RecQ DNA helicase family are widely present in organisms ranging from prokaryotes such as Escherichia coli (E. coli) to higher eukaryotes including humans. Conserved in the evolution process, these genes diversified along with the multicellularization of organisms. The E. coli RecQ gene was the first of the RecQ family genes to be discovered. This gene was identified as a gene participating in zygotic recombination and in the RecF pathway for UV damage repair (see Non-Patent Document 1). E. coli RecQ gene has been revealed to have the function of suppressing incorrect recombinations (see Non-Patent Document 2). The budding yeast SGS1 gene and the fission yeast Rqh1 gene are the only known RecQ homologues in these yeasts. Both of these genes mainly suppress recombination and play important roles in genome stabilization (see Non-Patent Documents 3 and 4). Higher eukaryotes carry a plurality of RecQ homologues. In humans, there are five types of genes known to belong to the RecQ family: the RecQL1 (see Non-Patent Document 6), BLM, WRN, RTS, and RecQL5 genes. Of these five, the RTS gene (see Non-Patent Document 5, and Patent Documents 1 and 2) and the RecQL5 gene (see Non-Patent Document 5, and Patent Document 3) were identified by the present inventors. BLM, WRN, and RTS genes respectively cause Bloom's syndrome (see Non-Patent Document 7), Werner's syndrome (see Non-Patent Document 8), and Rothmund-Thomson syndrome (see Non-Patent Document 9). These genes all play important roles in genome stabilization in cells.
In fibroblast cells and lymphocytic cell lines derived from patients with Werner's syndrome, chromosomal translocation and deletion, which are indexes for genome instability, have been reported to occur with a high frequency (see Non-Patent Document 10). Chromosomal breakage and sister chromatid exchange (SCE) are frequently detected in cells derived from patients with Bloom's syndrome (see Non-Patent Document 11). Trisomies of human chromosome 2 and 8 are frequently found in lymphocytes derived from patients with Rothmund-Thomson syndrome (see Non-Patent Document 12). These findings suggest that WRN helicase, BLM helicase, and RTS helicase encoded by the various causative genes of these three genetic diseases play important roles in genome stabilization in cells.
Telomere length abnormalities are seen in lymphocytic cell lines derived from patients with Werner's syndrome compared to cell lines derived from normal healthy subjects (see Non-Patent Document 13). In addition, cell immortalization was not observed in lymphocytic cell lines derived from patients with Werner's syndrome, although about 15% of cell lines derived from normal healthy subjects were immortalized after passaging (see Non-Patent Document 14). This finding indicates that WRN helicase contributes to telomere structure maintenance, and is thus essential for the immortalization (canceration) of lymphocytic cell lines.
It has been suggested that WRN helicase is associated with homologous recombination-mediated repair, because the helicase forms foci in the nucleus in response to DNA-damaging agents, and these foci are co-localized with the single-stranded DNA-binding protein RPA (which is a WRN-binding protein) and with the recombination repair factor RAD51 (see Non-Patent Document 15). In addition, WRN helicase has been known to bind to the DNA-dependent protein kinase complex (DNA-PK) and to flap endonuclease 1 (FEN-1). By binding to DNA-PK, WRN helicase plays an important role in the processing of terminals generated by DNA double strand breaks, which are repaired in the pathway of non-homologous end joining (see Non-Patent Document 16). WRN helicase is believed to activate FEN-1 by binding to it, and to provide a site for precise reconstruction of the replication fork through homologous recombination by processing Okazaki fragments (see Non-Patent Document 17). The above findings suggest that WRN helicase plays an important role in DNA repair during DNA replication.
BLM helicase is localized in the PML body, a specific structure found in the nucleus, and it binds to topoisomerase III (see Non-Patent Document 18). The helicase has the unwinding activity of the G-quadruplex structure, and thus is considered to contribute to telomere maintenance (see Non-Patent Document 19). Furthermore, the helicase has been reported to unwind the Holliday junction and to interact with the Rad51 protein (see Non-Patent Document 20). These findings suggest that BLM helicase cooperates with other DNA-metabolizing enzymes and plays important roles in recombinational DNA repair and telomere maintenance.
Of the five human proteins belonging to the RecQ DNA helicase family (RecQ1, BLM, WRN, RTS, and RecQ5), RecQ1, BLM, WRN, and RTS are expressed at negligible levels in resting cells, whereas they are expressed at high levels in cells whose proliferation has been enhanced by transformation with viruses (see Non-Patent Document 21). Furthermore, when the carcinogenic promoter TPA is added to resting cells, the expression of RecQ1, BLM, WRN, and RTS is induced along with the induction of cell division (see Non-Patent Document 21). These findings suggest the importance of the RecQ DNA helicase family in cell proliferation.
Taken collectively, these findings suggest that the RecQ DNA helicase family members may be potential target molecules for anti-cancer therapy because the family members participate in genomic repair in cells (BLM, WRN and RTS) and also the maintenance of telomere structure (BLM and WRN), play important roles in the immortalization of certain cells (WRN), and their expression is induced following cell division (RecQ1, BLM, WRN and RTS).
In addition, the expression levels of genes in the RecQ DNA helicase family are markedly higher in tumor cells. Thus compounds that suppress tumor growth can be screened using the suppression of expression of RecQ DNA helicase family genes as an index (see Patent Document 4). It has also been suggested that compounds suppressing RecQ helicase gene expression may suppress cancer cell growth (see Patent Document 4).
However, no one has reported the correlation between the suppression of RecQ DNA helicase family gene expression and the cancer cell-specific induction of apoptosis, and there have been no findings that suggest such a correlation.    [Patent document 1] Japanese Patent Application No. Hei 9-200387    [Patent document 2] Japanese Patent Application No. Hei 11-11218    [Patent document 3] Japanese Patent Application No. Hei 10-81492    [Patent document 4] Japanese Patent Application Kokai Publication No. (JP-A) 2000-166600 (unexamined, published Japanese patent application)    [Non-Patent Document 1] Nakayama, H., Nakayama, K., Nakayama, R., Irino, N., Nakayama, Y., and Hanawalt, P. C., “Isolation and genetic characterization of a thymineless death-resistant mutant of Escherichia coli K12: identification of a new mutation (recQ1) that blocks the RecF recombination pathway.”, Mol. Gen. Genet., 1984, Vol. 195, pp. 474-480.    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